With the growing demand for mobile broadband services and the appearance of new high-capacity mobile devices (e.g., smartphones, tablets) and applications, mobile networks users are requiring an ever increasing high-quality consumer experience, thus putting today's networks under tremendous pressure. Although the solutions currently under development (such as the Long Term Evolution-Advanced (LTE-A) standardized by 3GPP) represent a 4-times capacity increase over current 3 G systems, the envisaged band improvements will be inadequate to address all future requirements, making new approaches necessary. The ultra-high frequency (UHF, from 300 MHz to 3 GHz) spectrum which is currently allocated for mobile communications is becoming increasingly inadequate. In contrast, a large part of the spectrum in the 3-300 GHz frequency band, referred to as the millimeter-wave band (MMB), remains underutilized because of technical limitations, such as the stability of high radio frequency (RF) oscillators and the limited transmission range. The scenario of interest for the future is cellular networks that integrate the sub-3 GHz spectrum range with the new MMB.
Current RF equipment cannot manage the transmission and reception of wireless signals with frequencies ranging from few MHz to several GHz, thus each communication bandwidth requires the use of a specific RF transceiver, which means a specific cellular transmission system. Recently, photonic solutions have been proposed for generating phase-stable RF signals which avoid the up-conversion in noisy mixers at the wireless transmitter. As will be well known to the person skilled in the art, the heterodyne detection of two continuous wave (CW) optical signals in a photodiode generates a sinusoidal signal at their frequency difference, which can be used as an RF carrier. If one of the optical signals is also modulated, its modulation shape is transferred onto the beat signal directly at RF. When the two CW optical signals are phase-locked to each other the generated RF signal is particularly stable. This is the case when the CW optical signals are selected from the modes of a mode locking laser (MLL): the intrinsic phase-locking condition of the MLL ensures an extremely low phase noise in the generated RF signal, with excellent stability up to ultra high frequencies (up to W band and more) where performance of electronic generators strongly degrades, as reported by P. Ghelfi et al, “Generation of highly stable microwave signals based on regenerative fiber mode locked laser”, Conference on Lasers and Electro-Optics (CLEO), paper JWA47, 2010.